The present invention is a method and apparatus for treating cancer by inserting radioactive seeds into a patient for a possible curative effect. Permanent implant brachytherapy is a radiation treatment technique in which radioactive sources are implanted directly into the tumor and left in place permanently. Typically 50 to 100 small radioactive sources (“seeds”) are implanted in or near the tumorous tissue. The seeds may be a radioactive material contained within a biologically inert capsule. The seeds typically have a half-life between 2 and 75 days providing an average energy of emitted photons from approximately 25 keV to 500 keV, with a commercial source strength ranging from approximately 0.2 to several mCi.
The seeds are of a size, e.g., 0.8 millimeters in diameter and 3.5 to 5 millimeters long, so that they may be implanted with hollow needles. The needles have an outer diameter of 1.3 to 1.5 millimeters, are about twenty centimeters long, and have an inner diameter large enough for the seeds to pass. The seeds may be inserted preloaded in the needle along with spacers controlling their separation. With this approach, the loaded needle is inserted into the patient and then withdrawn while the contained seeds are held in place (effectively, ejected) by a stylet (plunger). Alternatively, the seeds may be dropped one at a time through the needle, either by hand or through the assistance of a device such as the Mick Applicator.
The most common application of permanent implants is in treatment for cancer of the prostate. Placement of the seeds for use in treatment of the prostrate may be done transperineally, whereby the needle is guided by a plate having predrilled holes at the intersection of a rectilinear grid with 0.5 cm spacing. The depth of insertion of the needle is confirmed by an image obtained with a transrectal ultrasonic image. In this way, seeds may be accurately placed at selected regular grid locations in a volume. U.S. Pat. (WARF No. 960296.99349) describes a planning method for determining the necessary locations of the seeds to produce a particular pattern of radiation dose and is hereby incorporated by reference.
The hole spacing in the guide-plate influences dosage to the cancerous and normal tissue. Improvement in the dose distribution can be achieved by reducing the spacing to less than 0.5 cm. A reduction in the current hole spacing is not amenable to the equipment used or operating room procedures, however.